Hepatitis B virus infection is a major health problem worldwide. HBV is a causative agent of both an acute and chronic form of hepatitis. More than 300 million people throughout the world are chronic carriers of HBV. Typically, the human host is unaware of infection and HBV infection leads to acute hepatitis and liver damage, abdominal pain, jaundice and elevated blood levels of certain enzymes. Additionally, HBV contributes to the formation of hepatocellular carcinoma and is second only to tobacco as a cause of human cancer. The mechanism by which HBV induces cancer is unknown, although it has been postulated that it may directly trigger tumor development or indirectly trigger tumor formation through chronic inflammation, cirrhosis and cell regeneration associated with the infection.
HBV belongs to the genus of hepadnavirus in the hepadnaviridae family. The genome of HBV consists of circular, partially double-stranded DNA of about 3200 base pairs that code for seven viral proteins. The polymerase gene completely overlaps the viral envelope genes PreS1, PreS2 and S and partially overlaps the X and core genes. The envelope of HBV consists of three proteins and their glycosylated derivatives. The three proteins, the small (S), middle (M) and large (L), are hepatitis B surface (HBs) proteins that contain the S gene sequence. Other proteins such as the MHBs contain the PreS2 sequence.
The core gene contains the nucleocapsid protein (183-185 aa) and the hepatitis B core antigen. The precore region, upstream of the core region, consists of 87 nucleotides that codes for 29 amino acids and is in phase with the core region. The first 19 amino acids of the precore region act as a signal for membrane translocation and eventual secretion of the precore gene product, the Hbe antigen.
Although a DNA virus, HBV requires reverse transcription through the formation of a (+) strand RNA intermediate. Reverse transcriptase has poor proof-reading ability and this leads to a high rate of nucleotide misincorporation; calculations suggest that this error-prone replication leads to one point replacement, deletion or insertion per 1000 to 100,000 nucleotides copied.
Additionally, other mutations in the precore, core and in the PreS and S, also give Hepatitis B virus a selective advantage in acquiring drug resistance.
The best defense to date has been vaccination. Human serum-derived vaccines, through genetic engineering, have been developed. Although the vaccine has been found effective, production has been hampered by the limited supply of human serum from chronic carriers and a long and expensive purification process. Furthermore, each batch of vaccine must be tested in chimpanzees to ensure safety. Additionally, vaccines do not help the patients already infected with the virus.
Great efforts have been made to develop clinically useful treatments for hepatitis B but they have been met with limited success. For example, interferon and several nucleoside analogs have shown relatively low cure rate of hepatitis B and they have often produced serious adverse effects. 2',3'-dideoxycytidine (ddC) has had high toxicity on the central and peripheral nervous system. Another nucleoside analog, ara-AMP, was found to transiently suppress HBV infection but also has been shown to be extremely toxic as well.
Cyclopentyl purine derivatives have also shown anti-viral activity. The process for preparing such compositions have been disclosed in U.S. Pat. Nos. 4,999,428 and 5,015,739. Additionally, Onishi et al., in U.S. Pat. No. 5,777,116 have disclosed a method of making cyclopropane derivatives that include a xanthin-9-yl group.
Schinazi et al., in U.S. Pat. No. 5,684,010, have produced enationtiomerically pure beta-D-dioxolane nucleosides which show selective anti-hepatitis B activity. Additionally, Lin et al., in U.S. Pat. No. 5,830,881, have discovered that certain dideoxynucleoside analogs which contain a ribofuranosyl moiety having a L-configuration instead of the usual D-configuration showed potent inhibition of viral replication. However, unlike other nucleoside analogs, these analogs have shown very low toxicity to the host cells such as animal or human.
In the past, anti-HBV nucleotide analogs such as 3TC, PCMEA, amd PCV have been used in clinical trials. However, some HBV-infected patients often experience a recurrence of HBV after a period of treatment with 3TC or PCV; this recurrence is due to the emergence of viral resistance. Additionally, the 3TC-resistant HBV, for example, becomes cross-resistant to other anti-HBV nucleotide analogs.
Hostetler, in U.S. Pat. No. 5,817,638, have produced nucleoside analogs such as 2',3'-dideoxycytosine, which are liked through a 5' phosphate of the pentose group to selected lipids such as dioleoylphosphatidylcholine. The lipophilic nature provides an advantage over the use of the nucleoside analog alone and makes it possible to incorporate them into the lamellar structure of liposomes. This form enables them to be taken up by liver cells which harbor the hepatitis B virus.
Processes have been disclosed by Holly et al., in U.S. Pat. Nos. 5,142,051, 5,641,763 and 5,869,467 for the production of N-(2-phosphonylmethoxyethyl) and N-(3-hydroxy-2-phosphonylmethoxypropyl) derivatives of pyrimidine and purine bases. These compounds also could include a xanthin-9-yl group. These compounds are regarded as acyclic analogues of nucleosides in which the nucleoside sugar moiety is replaced by a substituted carbon chain bearing hydroxy groups.
Although the compounds developed by Holly and others have not been tested specifically against hepatitis B viruses, they have shown in-vitro anti-viral activity against other DNA viruses such as the herpes viruses. Other analogs that show anti-Hepatitis B virus activity include phosphonomethyoxymethyl purine and pyrimidine derivatives as described by Kim et al in U.S. Pat. Nos. 5,726,174 and 5,837,871.
Chang et al., on the other hand, in U.S. Pat. No. 5,929,038, have developed an anti-HBV compound that is an iridoid aglycone compound produced from the parent iridoid glycosides which are monoteropenoid compounds and are derived from medicinal plants. In addition to inhibiting HBV DNA synthesis, these compounds also protect the liver from hepatic damage isuch as that induced by carbon tetrachloride intoxication.
In the past, anti-HBV nucleotide analogs such as 3TC (L(-)SddC), PCMEA, amd PCV have been used in clinical trials. However, some HBV-infected patients often experience a recurrence of HBV after a period of treatment with 3TC or PCV; this recurrence is due to the emergence of viral resistance. Additionally, the 3TC-resistant HBV, for example, becomes cross-resistant to other anti-HBV nucleotide analogs.
Flaviviruses belong to the genus Flavivirus of the family Togaviridae. According to virus taxonomy, about 50 viruses including Hepatitis C virus (HCV), Yellow Fever virus, Dengue Virus, Japanese Encephalitis virus, West Nile virus and related flaviviruses. The viruses belonging to the genus Flavivirus are simply called flaviviruses.
The flaviviruses are agents of infectious disease and predominate in East, Southeast and South Asia and Africa, although they may be found in other parts of the world as well. Japanese encephalitis virus is the causative agent of Japanese encephalitis (JE). The mortality rate from JE is rather high and the disease brings heavy sequelae. Although found in Japan, the disease has spread to other parts of Asia and is now found predominantly outside of Japan, primarily in South and Southeast Asia.
Dengue viruses are causative agents of dengue fever/dengue hemorrhagic fever. Infection with dengue viruses is a major public health problem in tropical countries, expecially in Southeast Asia and the Western Pacific, but dengue viruses may also be found in the Americas. As the dengue virus is transmitted to humans via the Aedes aegypti mosquito, it is not unexpected that the tropical and subtropical countries, in particular, those in Southeast Asia are highly endemic for dengue.
A major concern and increasing problem for public health officials has been the occurrence of severe complications which arise from dengue viral infections. Both dengue hemorrhagic fever (DHF) and shock syndrome (DSS) are clinical outcomes related to the presence of pre-existing immunity to a heterologous dengue virus serotype. Dengue hemorrhagic fever is initially characterized by a minor febrile illness lasting approximately 3-5 days. The patient may deteriorate at defervescence into the next phase of the syndrome with hemostatic disorders and increased vascular permeabilty frequently accompanied by internal bleeding and shock. As many as 1.5 million children are reported to have been hospitalized with 33,000 deaths from this syndrome since it was first recognized in Thailand in the 1950's. DHF/DSS has since continued to persist in South Asia. DHS/DSS is also found in a number of tropical or near tropical countries, including Cuba, Burma, Indonesia, India, Maldives, Sri Lanka and the South Pacific Islands. Dengue outbreaks are usually associated with a density of mosquito vectors, in particular, Aedes aegypti.
Dengue viruses can be divided into 4 serotpyes which are antigenically very similar to each other, but which differ enough to elicit only partial cross-protection after infection by one serotype. Such an infection by one serotype therefore, does not provide life-long immunity to the other serotypes. Vaccine approaches to preventing dengue infections have been unsuccessful to date.
Acute viral hepatitis is a disease which many result in chronic liver damage. It is clinically diagnosed by a well-defined set of patient symptoms, including jaundice, hepatic tendrness and an increase in the serum levels of alanine aminotransferase and aspartate aminotransferase. For many years, the agent of non-A, non-B hepatitis (NANBH) remained elusive. It has now been established that many cases of NANBH are caused by a distinct virus termed hepatitis C virus (HCV).
Colacino et al. in U.S. Pat. Nos. 5,821,242 and 5,891,874, have developed a series of benzimidazaole compounds which inhibit replication in other flaviviruses such as Hepatitis C by interfering with the structure and function of the viral replication complex.
To this date, no such research has involved the development of anti-HBV compounds from the helioxanthin class. Helioxanthin No. 145 has been isolated from the plant, Taiwaia cryptomeriodides.